How does sleep affect our body?

Sleep is often seen as a mere rest phase, in which our body simply switches off. But in fact, sleep is much more than that. While we sleep, our body performs a whole range of complex tasks. It's the time of the day when it regenerates and gathers new energy for the coming day. We sleep so that we can recover and the mind can process the experiences of the day. But what exactly does our body do while we slumber peacefully?